Such a hydraulic valve arrangement is, for example, known from DE 199 31 142 A1. Therein, one of the working connections is pressurised by means of a pressure medium and a common control valve, while the other working connection is relieved to a tank. In order to open the check valve of the working connection relieved to the tank, the pressure relief valve allocated to this check valve is subjected to a control pressure. This causes the check valve to be held in the closed position merely by the pressure of a spring, so that a small pressure increase at the working connection is sufficient to open the check valve.
In a float position of the control valve, both pressure relief valves are subjected to a control pressure, so that both working connections are relieved to the tank.
Such hydraulic valve arrangements are, for example, used with working vehicles that can be provided with working attachments. Attachments are, for example, snow clearance or street cleaning devices, but also agricultural devices.
Thus, such vehicles can be provided with different attachments. It is possible to connect both double-actuated consumers and single-actuated consumers. Consumers are, for example, single-acting or double-acting lifting mechanisms—so called hitches. With double-actuated hitches, a pressure medium actuates both the raising and the lowering movement. In this configuration the pressure increase occurring at the passive working connection of a hydraulic valve arrangement as described in the introduction will be sufficient to open the corresponding check valve.
With single-actuated hitches, in which a lift cylinder is pressurised by means of a pressure medium only during the raising movement, and the lowering movement is caused merely by the dead weight of the connected attachment, the weight of the connected attachment must be sufficient to provide a pressure increase that ensures the opening of the corresponding check valve. With light-weight attachments, it may happen that the pressure increase generated by gravity is not sufficient to open the check valve.
DE 102 24 827 A1 discloses a hydraulic valve arrangement with two working connections, a check valve being allocated to each working connection. A tappet is arranged between the valve elements of the two check valves, said tappet being subjected to pressure during the activation of one of the working connections in such a manner that it moves in the direction of the check valve of the other working connection, thus opening this check valve. This causes a direct relief to tank of the working connection that is not pressurised by a pressure medium. Thus, a reliable lowering is also ensured with single-acting hitches with light-weight attachments. However, with this valve arrangement it is not possible to assume a pressure-less float position, in which both check valves are open. A float position is for example advantageous, if the hitch has to be moved by external forces, which is, for example, the case, if an attachment has to follow the ground.
U.S. Pat. No. 3,908,515 discloses a hydraulic valve arrangement, in which a float position is enabled in that two tappets are arranged between the check valves, the tappet sides which face away from the check valves ending in a common pressure chamber. When the pressure chamber is pressurised, the two tappets are pressed away from each other and the check valves are opened. However, the pressurisation of the pressure chamber between the two tappets requires an additional valve arrangement, which increases the space requirement and the manufacturing costs. At the same time, the reliability of the valve arrangement is reduced.